Fuel cells are receiving considerable attention due to their direct and potentially highly-efficient generation of electrical power through the electrochemical conversion of hydrogen and other fuel sources without direct combustion.
In a typical fuel cell, an oxygen-containing gas (such as air) enters the fuel cell on the cathode side of the cell. At the cathode, oxygen in the air is converted (reduced) to oxide ions, which cross an electrolyte (typically a ceramic membrane) to the anode. On the anode side, the fuel is electrochemically oxidized producing both heat and electrical energy.
In a hydrogen-based fuel cell, for example, hydrogen is oxidized to water (steam). In a hydrocarbon-based fuel cell, the hydrocarbon (such as methane) is oxidized to carbon dioxide and water.
One issue with such fuel cells is that the oxidative reactions are highly exothermic, and the ability (or inability) to efficiently utilize this generated heat is directly related to the overall efficiency of the fuel cell process.
One proposed use of this excess thermal energy involves feeding a mixed hydrogen/methane fuel stream to the fuel cell. The hydrogen oxidation is the preferential reaction, and the heat generated at least partially causes the methane to reform (with steam) to hydrogen and carbon monoxide (which reaction is highly endothermic). The resulting hydrogen (and carbon monoxide) from the reforming reaction can then at least in part be consumed as part of the hydrogen fuel cell conversion.
One problem with the use of methane/hydrogen co-feed streams is that a higher fuel value off gas may be produced (containing significant amounts of methane, hydrogen and carbon monoxide). This off gas may be combusted for heat value (for example, to generate steam and electricity), but this is a relatively inefficient use.
In addition, both hydrogen and methane are value-added gases that have substantial other uses. A suitable feed stream for the fuel cell can be generated through the mixing of separately-generated hydrogen and methane streams, but this is not efficient. Therefore, a need remains for low-cost suitable feed streams for fuel cell use.
In view of numerous factors such as higher energy prices and environmental concerns, the production of value-added gaseous products (such as hydrogen and methane) from lower-fuel-value carbonaceous feedstocks, such as petroleum coke, coal and biomass, is receiving renewed attention. The catalytic gasification (hydromethanation) of such materials in the presence of a catalyst source and steam at elevated temperatures and pressures to produce methane, hydrogen and other value-added gases is disclosed, for example, in U.S. Pat. Nos. 3,828,474, 3,998,607, 4,057,512, 4,092,125, 4,094,650, 4,204,843, 4,468,231, 4,500,323, 4,541,841, 4,551,155, 4,558,027, 4,606,105, 4,617,027, 4,609,456, 5,017,282, 5,055,181, 6,187,465, 6,790,430, 6,894,183, 6,955,695, US2003/0167961A1, US2006/0265953A1, US2007/0000177A1, US2007/0083072A1, US2007/0277437A1, US2009/0048476A1, US2009/0090056A1, US2009/0090055A1, US2009/0165383A1, US2009/0166588A1, US2009/0165379A1, US2009/0170968A1, US2009/0165380A1, US2009/0165381A1, US2009/0165361A1, US2009/0165382A1, US2009/0169449A1, US2009/0169448A1, US2009/0165376A1, US2009/0165384A1, US2009/0217582A1, US2009/0220406A1, US2009/0217590A1, US2009/0217586A1, US2009/0217588A1, US2009/0218424A1, US2009/0217589A1, US2009/0217575A1, US2009/0229182A1, US2009/0217587A1, US2009/0246120A1, US2009/0259080A1, US2009/0260287A1, US2009/0324458A1, US2009/0324459A1, US2009/0324460A1, US2009/0324461A1, US2009/0324462A1 and GB1599932.
The hydromethanation of a carbon source to methane typically involves four separate reactions:Steam carbon: C+H2O→CO+H2  (I)Water-gas shift: CO+H2O→H2+CO2  (II)CO Methanation: CO+3H2→CH4+H2O  (III)Hydro-gasification: 2H2+C→CH4  (IV)
In the hydromethanation reaction, the first three reactions (I-III) predominate to result in the following overall reaction:2C+2H2O→CH4+CO2  (V).
The overall hydromethanation reaction is essentially thermally balanced; however, due to process heat losses and other energy requirements (such as required for evaporation of moisture entering the reactor with the feedstock), some heat must be added to maintain the thermal balance.
The reactions are also essentially syngas (hydrogen and carbon monoxide) balanced (syngas is produced and consumed); therefore, as carbon monoxide and hydrogen are withdrawn with the product gases, carbon monoxide and hydrogen need to be added to the reaction as required to avoid a deficiency.
In order to maintain the net heat of reaction as close to neutral as possible (only slightly exothermic or endothermic), and maintain the syngas balance, a superheated gas stream of steam, carbon monoxide and hydrogen is often fed to the hydromethanation reactor. Frequently, the carbon monoxide and hydrogen streams are recycle streams separated from the product gas, and/or are provided by reforming a portion of the product methane. See, for example, U.S. Pat. Nos. 4,094,650, 6,955,595 and US2007/083072A1.
The result is a “direct” methane-enriched raw product gas stream also containing substantial amounts of hydrogen and carbon monoxide which, after certain initial processing, is a potentially advantageous stream for use as a feed for fuel cells.
This potentially advantageous combination of hydromethanation and fuel cells has been recently recognized in, for example, “Integrated Gasification Fuel Cell Performance and Cost Assessment”, DOE/NETL-2009/1361 (Mar. 27, 2009). Two integrated configurations are proposed in that publication—both of which combust the anode output of the fuel cell for additional electrical power generation in a similar manner to that found in many Integrated Combined Cycle Gasification (“IGCC”) processes, and both of which utilize a variation of the hydromethanation process where oxygen is fed to a hydromethanation reaction for in situ generating syngas and heat required to keep the hydromethanation process in thermal and syngas balance (removing the need to recycle syngas). The proposed configurations in the publication, however, require significant quantities of gaseous oxygen, which is provided via conventional air separation technologies that are highly inefficient and can result in a significant drag on the overall process efficiency.
Therefore, a need remains for improved integrated hydromethanation fuel cell processes with higher efficiencies, for example, through the reduced use of oxygen in the process.